As a technique relating to control of a hydraulic control valve for use in a valve timing control apparatus for an internal combustion engine, for instance, Patent Document 1 identified below discloses a technique relating to a control apparatus as described below. Namely, this control apparatus is a duty-ratio control apparatus operable to control an amount of power supplied to an electromagnetic actuator by varying a ratio between an ON duration and an OFF duration in one duty cycle. In operation, this control apparatus effects control operations for decreasing one cycle period (increasing the PWM frequency) with decrease in the power supply amount to the electromagnetic actuator and increasing the one cycle period (decreasing the PWM frequency) with increase in the power supply amount to the electromagnetic actuator.
This control apparatus is intended to adjust dither oscillation of the spool (valve member) of the electromagnetic actuator within a predetermined range even when there occurs change in the condition of the electromagnetic force acting on this spool due to variation in the amount of power supplied thereto. More particularly, when the power supply amount to the electromagnetic actuator is small, the one cycle period is reduced thereby to restrict the pulsate electromagnetic force due to the ON/OFF current so as to restrict increase in the dither amplitude, so that occurrence of hunting of the spool of the electromagnetic actuator may be restricted. Conversely, when the power supply amount to the electromagnetic actuator is large, the one cycle period is increased for increasing the pulsating electromagnetic force generated by the ON/OFF current, so as to maintain kinematic friction condition by dither oscillation of an appropriate amplitude, thus preventing deterioration in the responsiveness of the spool of the electromagnetic actuator.
Patent Document 1: Japanese Patent Application “Kokai” No. 2004-301224 (page 3, page 8, FIG. 1)